8UBMAU1NK TfXNKLIXG 257 



plates with cover-plate joints and interior angle stiffeners ; the 

 shell of the East River tunnel shield, 11 ft. in diameter, was 

 made up of one $-in. and one |-in. plate; the Blackwall tunnel 

 shield, 27 ft. 9 ins. in diameter, had a shell consisting of 

 four thicknesses of f-in. plates ; and the Clichy tunnel shield, 

 with a diameter of 2.06 meters, had a shell 2 millimeters thick. 

 Front-End Construction. By the front end is meant that 

 portion of the shield between the cutting-edge and the vertical 

 diaphragm. The length of this portion of the shield was 

 formerly made quite small, and where the material penetrated 

 is very soft, a short front-end construction yet has many advo- 

 cates ; but the general tendency now is to extend the cutting- 

 edge far enough ahead of the diaphragm to form a fair-sized 

 working chamber. Excavation is far more easy and rapid when 

 the face can be attacked directly from in front of the diaphragm 

 than where the work has to be done form behind through the 

 apertures in the diaphragm. So long as the roof of the excava- 

 tion is supported from falling, experience has shown that it is 

 easily possible to extend the excavation safely some distance 

 ahead of the diaphragm. In reasonably stable material, like 

 compact clay, the front face will usually stand alone for the 

 short time necessary to excavate the section and advance the 

 shield one stage. In softer material the face can usually be 

 sustained for the same short period by means of compressed air; 

 or the face of the excavation, instead of beinu r made vertical, can 

 be allowed to assume its natural slope. In the latter case a 

 visor-shaped front-end construction, such as was used on some 

 portions of the Clichy tunnel, is particularly advantageous. The 

 following figures show the lengths of the front ends of a number 

 of representative tunnel shields. 



Pity and Pontli London . 1 ft. Merer y Kivor 3 ft. 



urltlver .... 11.2.1 " Kant River 31 " 



Hudson Rivrr .... 6! Rlnrkwall 6.5 " 



Two general types of construction are employed for the 

 cutting-edge. The first type consists of a cnst-iron or oatt-lteel 



